Dendritic cells (DCs) play critical roles in tolerance as well as in immunity. Detailed understanding of factor(s) and condition(s) leading to such divergent states of DCs is critical in understanding immune responses against pathogens and tumors, as well as self antigens DCs express pattern recognition receptors such as Toll-like receptors (TLRs) on the cell surface. At least 10 TLRs with distinct ligand binding specificities have been identified. Although TLRs are evolved to sense microbial products, the existence of endogenous TLR activators has been proposed. Recent studies suggest that TLR2 and TLR4 are also the receptors for a ubiquitous mammalian heat shock protein gp96 of the endoplasmic reticulum. The presence of cell surface receptor for an intracellular molecule suggests that the export of gp96 from cells must be regulated in vivo and uncontrolled gp96 export must have immunological consequences. This proposal aims to address the immunological significance of cell surface gp96 in immunity on an organismal level, by using novel transgenic mouse models where gp96 is targeted to cell surfaces broadly. We demonstrated that surface translocation of gp96 in vivo triggers a MyD88-dependent systemic autoimmunity, which provides perhaps the strongest evidence yet for gp96 as an endogenous proinflammatory molecule. Two broad hypotheses will be addressed. First, chronic stimulation of DCs bygp96 breaks peripheral T cell tolerance and induces autoimmune diseases. Second, gp96 export is critically important for the regulation of T cell immunity. This line of work should contribute to the increasing appreciation that the immune system evolves and functions not only against exogenous pathogen-associated molecular patterns, but also towards endogenous "danger molecules" of the host. Our proposal therefore should have implications in cancer immunity as well as in autoimmune diseases. Specifically, we aim to: (1) determine if disruption of gp96 retention mechanisms induces DC activation and exacerbates autoimmunity;(2) study the impact of cell surface gp96 on T cell activation and memory; and (3) elucidate the roles of cell surface gp96, in peripheral tolerance, against self-reactive T cells otherwise destined to anergy/tolerance.